Investors have thrown a record $49 billion into junk bond mutual funds and exchange-traded funds so far this year, more than twice the full-year record of $21 billion for 2009, according to EPFR Global.

"That alone is indicative of an overheating type of market," George Rusnak, director of fixed income at Wells Fargo, tells CNNMoney.

The key for junk bond investors is to determine which companies will be able to pay their debts, Gramatovich says. "It does appear conservatism is warranted."

Beware particularly of bonds that allow companies to pay interest in more bonds rather than in cash if the companies are having financial difficulties.

“It is an expression by the company that we’re not sure we’re going to be making enough money to pay the interest for the next few years,” Adam Cohen, founder of Covenant Review, a credit research firm, tells The New York Times.

High-yield (junk) bonds have been on a roll for the past 3 ½ years, and that has yield-starved investors rushing into the market. However, some experts are worried that it’s all gone too far, as weaker companies are now able to issue paper, and investors may get stuck with some duds.